The field of this invention is nuclear magnetic resonance (NMR) imaging methods, and particularly, the acquisition of images of an object which functions in a cyclic pattern such as the human heart.
NMR has been developed as an imaging modality which is utilized to obtain images of anatomical features of human patients. Such images depict distributions which are dependent upon nuclear spin density (typically, protons associated with water and tissue), spin-lattice relaxation time T.sub.1, and/or spin-spin relaxation time T.sub.2, and these are believed to be of medical diagnostic value in determining the state of health of the tissue examined. Data for constructing NMR images can be collected using one of many available techniques, such as multiple angle projection reconstruction and Fourier transform (FT). Typically, such techniques comprise a series of pulse sequences. Each pulse sequence comprises at least an RF excitation pulse which produces transverse magnetization in the processing nuclei, and a magnetic field gradient pulse which encodes spatial information into the resulting NMR signal. As is well known, the NMR signal may be a free induction decay (FID) or, preferably, a spin-echo signal. The NMR signals from the pulse sequences are processed to produce the desired image.
The preferred embodiments of the invention will be described in detail with reference to a variant of the FT technique, which is frequently referred to as "spin warp." It will be recognized, however, that the method of the invention is not limited to FT imaging technique, but may be advantageously practiced in conjunction with other techniques. The spin-warp technique is discussed in an article entitled "Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein, et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). Briefly, the spin-warp technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this encoding gradient. In a two-dimensional implementation (2DFT), spatial information is encoded in one direction by applying a phase-encoding gradient along that direction and then observing a spin-echo signal in the presence of a magnetic field gradient in a direction orthogonal to the phase-encoding direction. The gradient present during the spin echo encodes spatial information in the orthogonal direction. In a typical 2DFT data acquisition procedure, the magnitude of the phase-encoding gradient pulse is incremented monotonically in each successive pulse sequence so as to methodically produce NMR data which represent samples of the Fourier transform of the entire distribution to be imaged. Typically, 128 or 256 such sequences are required, the number depending on the desired spatial resolution and field of view in the phaseencoding direction.
Although it has been known that some NMR imaging pulse sequences produce artifacts due to object motion, early in the development of NMR imaging it was believed that among the advantages of the FT imaging method was its property of not producing motion artifacts. However, it is now well recognized that this is not so. Object motion during the acquisition of an NMR image produces both blurring, streaking and "ghosts" in the phase-encoded direction. Ghosts are particularly apparent when the motion is periodic, or nearly so, whereas streaks derive from random motion. For most physiological motion, including cardiac and respiratory motion, each NMR spin-echo or FID can be considered a snap-shot view of a portion of the object's Fourier transform. Blurring and ghosts are, therefore, due to the inconsistent disposition of the object from view to view rather than from motion during the view acquisition.
Both delecterious effects of periodic motion, blurring and ghosts, can be reduced if the data acquisition for each sequence is synchronized with the periodic motion. This method is known as gated scanning. Conventional gated cardiac NMR imaging techniques use a standard pulse sequence to gather data in synchrony with each heartbeat. The beginning of each data acquisition sequence is triggered by a programmed delay following the detected peak of a signal produced by the cardiac "QRS" wave complex. Thus, each heartbeat produces one view (phase encoding value) of the data set. After typically 128 or 256 heartbeats, sufficient data is available to produce the image. Because each data acquisition occurs when the heart is in the same phase of its motion cycle, the image so formed should represent an accurate picture of the heart at the selected point in its functional cycle. By altering the programmed delay time between the QRS peak and the commencement of the sequence, images may be formed of different phases of the cardiac cycle.
Unfortunately, the periodicity of the human heart is imperfect and the data acquired during successive heart beats may, in fact, capture the heart at slightly different phases of its cycle. The reconstructed image, therefore, will be somewhat degraded by blurring and other artifacts which result from view-to-view inconsistencies.
In addition to producing single images of the heart at specific phases of its cycle, there is significant medical value in producing a series of images which depict the heart at successive phases of its cycle. Indeed, a motion picture, or cine, of the heart cycle is desired. To minimize the amount of time a patient must spend in the NMR imager in order to acquire the necessary data, it is imparative that data for more than one image be gathered during each heart cycle.
In one prior art method, this is accomplished by exciting the slice of interest several times at fixed time intervals of, for example, 200 milliseconds during each cardiac cycle and using the same phase encoding amplitude for each. The data is sorted and employed to produce a set of images, for example, 4 or 5 at different delays with respect to the QRS complex. The later images in the set, unfortunately, suffer more from the above mentioned problems since they are farther removed in time from the cardiac signal landmark. In fact, it is essentially impossible to reliably image the late stages of the cardiac cycle in the face of variability in the heart rate with this prospective triggering method.
Other problems with prior art methods result from the fact that the functional cycle of the heart is not perfectly periodic. When the NMR pulse sequences are executed in synchronism with the heart's functional cycle as taught by the prior art, their repetition rate varies along with the heart rate. Where the repetition rate is relatively high, as when acquiring NMR data for four or five images, the NMR equilibrium due to T recovery will not be stable due to these variations. As a result, the image produced for the early part of the cardiac cycle will be degraded and will have a different appearance than the later images.
Another problem caused by variations in the cardiac cycle is that a guard band of extra time must be provided at the end of each cycle. This guard band must be chosen long enough to insure that the last NMR pulse sequence is executed before the shortest expected cardiac cycle is completed. As a result, either the number of images to be acquired must be reduced, or the time interval between NMR pulse sequences must be reduced.